Electrical activity of the heart Flashcards
What is the combination of electrical and physical connections in cardiac muscle called?
Functional syncytium
What is cardiac muscle electrically connected by?
Gap junctions which are protein channels
What physically connects cardiac muscle?
Desmosomes
These form intercalated discs
What are T-tubules?
Deep invaginations of sarcolemma
What is the sarcoplasmic reticulum?
Essentially the calcium store
What do the intercalated discs do?
Allow the cardiac muscles to act like one cell even though each cell has its own nucleus
What is the sequence of events for contraction?
- Action potential enters via contractile cell and moves across sarcolemma and into t-tubules
- Calcium flows out of SR and into cytosol
- Diffuses across cytosol to the contractile elements
- Binds to troponin and initiates cross-bridge formation and sliding filament movement
What happens in relaxation?
- When cytoplasmic calcium concentrations decrease, calcium unbinds from troponin
- Myosin releases actin
- Contractile filaments slide back to their relaxed position
- Calcium is transported back to ST via Ca2+-ATPase
- Calcium is removed from the cell in exchange for Na+
How does the cardiac cell have a graded contraction?
- Force generated is proportional to number of active cross-bridges
- Number of cross-bridges active is determined by how much Ca2+ is bound to troponin
- If more calcium enters from the extracellular fluid, more Ca2+ is released from SR
- Myosin forms more cross-bridges with actin
- Additional contractile force
What is the resting membrane potential of a non-pacemaker cell? Why?
- 80mV
Due to high resting P_K+
Why does the initial depolarisation in a non-pacemaker cell occur?
Increase in P_Na+
Cardiac cells have high voltage gated sodium channels and open when there’s a huge depolarisation
Why does the action potential plateau in a non-pacemaker cell?
Increase in P_Ca2+ and increase in P_K+
The slower voltage gated calcium channels open
Potassium permeability also drops to allow for further depolarisation
What causes repolarisation in non-pacemaker cells?
Calcium channels shut and permeability increase for K+
Why is there a long refractory period in non-pacemaker cells?
Slower voltage gated sodium channels opening
Last for as long as muscle twich
Prevents tetanus as there’s no summation
What is initial membrane potential of a pacemaker cell?
It is -60 mV and gradually rises to a threshold of -40 mV
Why is the pacemaker potential so complex?
Gradual decrease in Pk+, leakiness decreases
Early increase in PNa+, increase in permeability opened by repolarisation from previous action potential
Late increase in PCa2+, tiny calcium channels that let calcium in
What are some examples of modulators of electrical activity?
- Sympathetic and parasympathetic systems
- Drugs
- Temperature
- Hyper/hypokalaemia
- Hyper/hypocalcemia
Specifically which drugs modulate electrical activity? What do they do?
Ca2+-channel blockers: decrease force of contraction
Cardiac glycosides: increase force of contraction, calcium
How does temperature alter electrical activity?
Increases by ~10 beats/min/°C
Ions pass through channels quicker
This is why there’s tachycardia in fever
How does hyperkalaemia modulate electrical activity?
High plasma K+
Fibrillation & heart block
What is fibrillation?
Depolarises cells
Action potentials fire spontaneously
Leads to uncoordinated firing
What is heart block?
Depolarised speed of propagation slows down
Can slow down so much that it stops
How does hypokalaemia modulate electrical activity?
Fibrillation and heart block
Starts to polarise and then depolarise
Same net effect as hyperkalaemia
How does hypercalcemia modulate electrical activity?
High plasma Ca2+
Increased HR and force of contraction
